1. Field of the Invention
The present invention relates generally to an inter-LAN (local area network) communication system wherein LAN terminals are interconnected by an ATM switch, and more specifically to an address updating technique for cell assembly disassembly facilities used between the ATM switch and the local area networks for adaptively updating the address information contained in the ATM cells received from the ATM switch.
2. Description of the Related Art
In a communication system in which local area networks such as Ethernet are interconnected by an ATM switch, the data handled by the LAN terminals are in the form of variable bit rate (VBR) packets, while the data handled by the ATM switch are in the form of fixed length ATM cells whose payload contains a source address (SA) and whose header contains a virtual path identifier (VPI) and a virtual channel identifier (VCI). A cell assembly disassembly facility is therefore provided at the interface between the ATM switch and each local area network to provide data format conversion between packets and ATM cells. ATM cells bearing the same address information are assembled into a packet, and an error check is performed on the packet using the CRC (cyclic redundant check) bits contained in the cells before the packet is sent to the local area network. The cell assembly disassembly facility includes an address table. If no error bits are detected by the CRC error check, the address table is searched for the same address information as that of the type of ATM cell such as BOM (beginning of message) or SSM (single segment message) cell. If there is none, the SA, VPI and VCI of the ATM cell is stored into the DA (destination address), VPI and VCI fields of a given entry together with a time-lapse indicator. If the same address information is already stored, the time-lapse indication of the registered address information is updated with that of the most recent BOM or SSM cell. The time-lapse indicators of all of the entries are incremented at periodic intervals. At longer intervals, the time-lapse indicators of all the entries are checked to erase those entries whose time-lapse indicators exceed a predetermined threshold to make them vacant for subsequent cells. On the other hand, when a packet is received from the local area network, the address table is searched for a DA field that matches the destination address of the packet. The packet is then assembled into one or more ATM cells each containing the VPI and VCI of the entry of the matched DA field.
Since the conventional cell assembly disassembly facility is required to make a search through the address table each time a CRC error check is made upon assemblage of a packet, the cell assembly disassembly facility is considerably overloaded.